texts: Is it possible for a septuple eclipse to happen on Saturn considering there are 7 moons capable of eclipsing the Sun?

texts:
The reason quadruple eclipses can never happen on Jupiter is because of the 1:2:4 orbital resonance between Io, Europa, and Ganymede. As far as I know this isn't a problem for Saturn's moons.

From Wikipedia's Solar eclipses on Saturn:

  Seven of Saturn's satellites – Janus, Mimas, Enceladus, Tethys, Rhea, Dione and Titan – are large enough and near enough to eclipse or occult the Sun, or in other words to cast an umbra on Saturn.

Of the 7 listed above, Mimas and Tethys are in a 2:1 orbital resonance and so are Enceladus and Dione, however that doesn't stop either pair (or all 4 together) from eclipsing Saturn at the same time.

So, in theory, it is possible for all 7 moons to more or less line up together. Does this mean that during eclipse season on Saturn, there can be a septuple eclipse? If so, when is the next one going to happen?


texts:
I have recently read a bit a out Bok globules. One thing, that is repeatedly emphasised is that they are among the coldest objects in space, with a temperature of ~3 K.

Why is that so? Naively, I would expect the coldest objects to be far out in the super voids. Also, I would expect these globules to have retained heat from the gravitational collapse. After all, the kinetic energy of their free fall should be dissipated, shouldn't it?

Maybe someone can elaborate.


texts:
Does every star become a black hole? Is there any probability that our sun can become a black hole? If yes then is it on its way to become a black hole? what is the current state of sun as per the black hole life cycle? What will be the effect on all the planetary objects in the solar system if the sun turns into a black hole.

Sorry for so many questions but i cannot miss them as these are some questions on my mind.


texts: H(t) - time graph can have positive gradient if acceleration is so big?

texts:
Does the Hubble constant depend on redshift?
please see above quastion and answer.
I wonder the below graph(from above question's answer)'s gradient can be positive if acceleration is so big.


texts: How (the heck) does Astrometry.net work?

texts:
What magic is this?

This answer to my question Astronomy detective question: what part of the sky are these photos of? What are a few of the stars? nails it, and the output (linked there) contains the following images along with a whole lot more information.

The images are analyzed by astrometry.net

How does it do what it does? If I understand correctly you can upload random star images with a wide variety of fields of view (from telescopes to cell phones) and random orientations and no further information, and it figures it out.

Is there a way to explain how the algorithm works with more detail than "a computer matches the dots to known stars"?

Related question about these cell phone images: Milky Way with a cell phone, how is this possible?


texts: New evidence against the existence of dark matter?

texts:
I've just read and heard about an article which gives new evidence for MOND - Modified Newtonian Dynamics, which is an alternative to dark matter by proposing that our gravitational law should be modified, and with it evidence against the dark matter hypothesis. I was first brought to attention on this by a video by Dr. Becky on Youtube:

Here is the article itself:
https://iopscience.iop.org/article/10.3847/1538-4357/abbb96
And a few other articles on the discovery:
http://www.sci-news.com/astronomy/modified-gravity-theory-09165.html
https://www.universetoday.com/149416/new-data-supports-the-modified-gravity-explanation-for-dark-matter-much-to-the-surprise-of-the-researchers/
https://newatlas.com/physics/modified-gravity-evidence-dark-matter/
Does anybody know more about this? Isn't this quite significant? Or am I missing something?


texts: How high does light pollution reach into the sky?

texts: I want to know how high does light pollution typically reach into the sky straight up? Has anyone ever researched this because I wonder if being at 10000ft elevation would reduce the amount of light pollution from nearby cities because of the height assuming that light pollution weakens the higher you go. How high would it go straight up the sky (not to the sides about how far I would have to travel to have no light pollution disturbing the night sky)?


texts: What does "synchrotron emission from relativistic electrons in equipartition with a ∼1.5 mG magnetic field" mean?

texts:
I've heard of the equipartition theorem in the context of thermodynamics.
But I don't understand how either "synchrotron emission" or "relativistic electrons" can be in "equipartition with a ∼1.5 mG magnetic field".
Does it mean that the energy density stored in the magnetic field is equal to (or is equilibrated with) the energy density of the relativistic electrons and/or the synchrotron radiation that they produce?
The phrase is from a block quote in this extensive & thorough answer to Nature and mechanism of the short-term variability in radio strength of Sgr A*? citing F. Yusef-Zadeh (2017) ALMA and VLA observations of emission from the environment of Sgr A*
I see "energy density" half-way through the paragraph below, so I think I may be on the right track, but this is over my head:

4 DISCUSSION
4.1 Low Extinction Millimeter Halo
[...]The millimeter halo is coincident with the X-ray emission around Sgr A*, which is dominated by bremsstrahlung arising from a medium with ne∼150 cm−3 and T∼3×107 K. The bremsstrahlung contribution at 230 GHz, about 0.2 µJy, is negligible. Thermal continuum from dust can also be ruled out because of the halo’s extinction deficit of 0.5 magnitudes at H-band relative to its surroundings. The millimeter emission could, however, be produced by synchrotron emission from relativistic electrons in equipartition with a ∼ 1.5 mG magnetic field. The energy density of each of these components would then be ∼ 10% of the thermal energy density of the hot gas, so this is plausible. The luminosity in the mm is $4 \pi d^2 \nu S_{\nu}$ ∼1.4×1033 erg s−1, comparable to the X-ray luminosity, LX ∼1×1033 erg s−1, implying that synchrotron cooling is marginally the dominant cooling mechanism for the gas. The synchrotron cooling time is ∼1000 yr, cf. the hot gas cooling time ∼105 yr, so this requires electron acceleration on this time scale.


texts: When is it a good aproximation to consider a star to be an ideal gas?

texts:
I am currently taking a first course on stellar astrophysics, and I noticed that in some cases we use the ideal gas equation of state for stars, so we also use $\gamma =5/3$. Of course it can only be applied where there is no nuclear reaction, so there is a limit in the temperature.

Also, if the radiation pressure if relevant, you have to consider the parameter $\beta$ to calculate the total pressure and the adiabatic coefficient $\gamma$. It is also incorrect (I think) if there are degeneration or relativistic considerations.

I don't know if I have to consider other factors before I can use this equation of state and value for $\gamma$, maybe it is important if it is a convective or radiative area, or other factors I didn't consider.

My question is: can anybody tell me the limits of the ideal gas approximation? (quantitative better than qualitative but any help will be well received)


texts: how do we know the universe is STILL expanding?

texts: According to Hubble more distant galaxies are moving away faster but since the light took billions of years to reach us we should say they WERE moving away faster billions of years ago. How do we know that those galaxies where light is just reaching us know are STILL moving away at all.... let alone faster


texts: 10 seconds before the M87 black hole was a black hole, what was it? A star 10 times the size of our solar system?


texts:
Some Background?
There are many types of Supernovae that can be classified into the following categories :-
(This list is available here.)
Type 1A supernovae

• Their spectra show very little hydrogen and a lot of carbon; they also show silicon, calcium, and elements up to iron (due to fusion during the intense explosions).

• Astronomers believe this type of supernova results from a white dwarf (the carbon-oxygen remnant of a once Sun-like star) that has collected too much material for its internal pressure to withstand. Once its mass reaches 1.4 times that of our Sun, the white dwarf collapses, which causes its internal temperature to rapidly rise. Then, the carbon fuses, and the entire star explodes as a carbon-detonation supernova. The initial white dwarf can collect material from a company red giant star or from colliding with another white dwarf.

Type 1B supernovae

• Their spectra contain very little hydrogen; they also show helium.

• Scientists think this type of supernova results from the death of a star at least 25 times the Sun’s mass. The star sheds material from its gaseous envelope late in its life (hence the lack of hydrogen in its spectrum). The star then implodes, bounces back, and explodes.

Type 1C supernovae

• Their spectra contain very little hydrogen or helium.

• Scientists believe this type forms like a type 1B supernova; collectively, these objects are called “stripped core-collapse supernovae.”

Type 2 supernovae

• This type of supernova has a lot of hydrogen and helium in its spectrum.

• Astronomers think this type results from the death of a star larger than eight times our Sun’s mass. Such a star can fuse elements up to iron. Its core then heats up, implodes, and bounces back to explode, spewing heavy elements into space. The enormous explosion leaves behind a neutron star or a black hole, depending on the mass of the initial star. Type II supernovae are called “core collapse supernovae.”

Hypernovae
(I found this here)

SUPERLUMINOUS SUPERNOVA (Hypernova): A burst 5 to 50 times more energetic than a supernova. A hypernova may or may not be associated with a powerful burst of gamma radiation.

So, What's the Question?
I was reading this article by NASA that showed that some scientists discovered what are suspected of being the remains of Hypernovae. This lead me to wonder how astronomers differentiate between Supernova remains and Hypernova remains.
Question(s) -

How do you know that the remnants of the explosion of a Star are of a Hypernova?
What techniques and equipment are used to do so?
What is the main difference between Supernovae and Hypernovae remains when it comes to observing them?


texts: How significant and accessible is modern Chinese astronomy?

texts:
Is China fully integrated in the international astronomical community, or does military secrecy and language problems and sanctions (ITAR) or something else stand in the way? Does China have valuable assets for astronomical observations, and a vibrant national astronomical academical community?

China has an ambitious and successful HSF and Lunar program with orbiters and a lander. They have also announced a very ambitious program for inter-planetary probes to practically the entire Solar system, to date they've flewn by the NEO Toutatis. The are also building a 500 meter radio telescope super-version of Arecibo.

Will for example US astronomers be able to use that telescope? Will they be offered samples of the upcoming Chinese lunar sample return, maybe from the lunar far side? Have lunar ground penetrating radar data from Chang'e 3 been presented publicly and led to papers in international journals? Is there any prominent Chinese astronomical journal?


texts: Is Beta Lyrae still thought to be losing mass in a spiral outflow?

texts:
In the famous 1960 painting by Chesley Bonestell, Beta Lyrae is depicted with a glowing spiral trail of matter emanating from the binary stars. I'm wondering if that's still thought to be a possibility given the additional study of the system between the 1960s and the present day.

Deschamps et al. (2005) suggest that non-conservative evolution of binary stars might be via a hotspot mechanism, where the mass is lost from the region where the accretion stream impacts the disc. Most studies of Beta Lyrae itself seem to discuss mass loss via bipolar jets, for which there is observational evidence (e.g. Ak et al. 2007). Is there any evidence for the kind of spiral outflow depicted by Bonestell (either via the Deschamps et al. mechanism or via direct loss through the L2 point), or is this no longer thought to be occurring at Beta Lyrae?


texts: Does matter accumulate just outside the event horizon of a black hole?

texts:
My understanding is that time slows and approaches stopping when approaching the event horizon of a black hole.  I have seen this explained several places, including a brief explanation in the last paragraph under: http://en.wikipedia.org/wiki/Black_hole#General_relativity, quoted below:

  Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars",[17] because an outside observer  would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius.

Does this mean then that no matter actually falls into a black hole (except possibly what was there at its formation)?  Would this also mean matter is accumulating just outside its event horizon?  As I understand it, this would be the perspective from outside the black hole.  If this is the case, I wonder if we would observe a tremendous amount of matter surrounding the event horizon, but it would be extremely red shifted?

Edit:

I noticed an answer to a different question, especially the end portion, provides some insight here as well:
https://astronomy.stackexchange.com/a/1009/1386

Edit:

These YouTube videos someone put together explain the concept very well, and seem to indicate this idea is gaining traction!

https://www.youtube.com/watch?v=yZvgeAbrjgc&list=PL57CC037B74307650&index=118
https://www.youtube.com/watch?v=b1s7omTe1HI

Edit:

This new YouTube video describes this idea very well, and describes it as the way black holes work!

https://youtu.be/mquEWFutlbs


texts: Location on (or in) the celestial sphere

texts:
First off, I apologize if I am breaking any site-etiquitte. Secondly, this is probably extremely elementary.

For a lab, I was given the question:

  When we are sitting in a planetarium, where are we located on(or in)
  the "celestial sphere"?

To me, this seems like a silly question - because the answer is really dependent on where the operator places you. Is this assumption wrong? Is there a particular place in the celestial sphere that you would be?

I also thought that it might just be the very center of the celestial sphere.


texts:
I would like to see some GEO and HEO images of Earth. But I can't find a good source. There are a lot of LEO images of Earth but I can'f find any raw images from higher (preferably 30,000km+) altitudes. There should be at least 400 satellites at that height, according to Wikipedia.

Is there any website with those kind of images?


texts:
At this point in time, evidence for the existence of dark matter has accumulated in many ways:

it affects galactic rotation curves
plays a major role in cosmology, and the evolution of structure in the universe
is predicted in copious amounts by gravitational lensing on a wide range of scales
influences the dynamics of galaxy clusters

to name a few.

There are many known candidates for dark matter particles: WIMPs, axions, WISPs, neutrinos, etc (in fact, even bricks, though some other considerations would exclude them).

The question then is:
Why do we expect that only one type of dark matter particles is responsible for phenomenological dark matter?

For example, $\Lambda$CDM cosmology, the standard cosmological model, requires dark matter to be cold (slow, non-relativistic), which is used to constrain the possible properties of dark matter particles. However, this doesn't actually imply, that dark matter is cold for all the astrophysical systems. For example, galactic halos could be made of warm dark matter, and halos of dwarf galaxies could be made of cold dark matter.

One might of course say that one-species model is the simplest one. The counter-argument would be that in reality there well may be many species. This in turn might have profound implications for astrophysical models.

To summarize the question: Is there any good reason, preferably supported by observations, to think that only one species of dark matter is present in all the models currently used?


texts: Strength of core-envelope coupling in stars (again)

texts:
I asked this on the physics SE but it received little attention:
Consider a high-mass zero-age main-sequence (ZAMS) (e.g., $m_{\rm ZAMS} \gtrsim 30\,$M$_{\odot}$) star.
I understand that the core-envelope boundary is only (semi-) well defined for Main Sequence stars that do have a core, and that in principle the final spin (angular velocity) of the core should depend on the spin of the star prior to losing its envelope due to strong wind mass-loss in late-stage evolution if the star is in isolation (or due to a mass transfer event if the star is in a binary). I've read, here for instance, that the core should be nearly completely uncoupled from the envelope, however in this recent paper they assume that the coupling is sufficient enough to determine the natal spin of the stellar core (see figure 1). Here, they treat core-envelope coupling of high mass stars as a great uncertainty. I understand that this coupling is uncertain (observationally poorly constrained) and is theoretically uncertain due to many complicated processes involved in sustaining the transfer of angular momentum through the star.
Is there a (preferably) simple way to understand the strength of core-envelope coupling in terms of the ZAMS quantities of the star, without performing full stellar population synthesis?... and without introducing another uncertain parameter, such as a timescale? Is there a known parameterization of the effects of this process on single-star evolution, i.e. with the ZAMS mass and metallicity?


texts: Opposite Constellation of Sagittarius A*

texts:
Suppose an alien race fired a laser beam from the centre of the Milky Way, it went through the Sun and then off further out of the galaxy.  What Constellation would the light beam head towards, closest star also if possible?  Am not interested about how much devastation the light beam may or may not cause, just the journey and destination.

Would that direction be the quickest/shortest way out of the galaxy if we had a spaceship that could travel that fast?


texts: Ceres is spherical, Vesta isn't – is it the largest non-spherical solid body in the solar system?

texts:
I was showing a recent video about Ceres to a friend and we looked up asteroids. Ceres has attained a fairly spherical shape, but Vesta still has a way to go. From Wikipedia:

  Vesta's shape is close to a gravitationally relaxed oblate spheroid, but the large concavity and protrusion at the southern pole (see 'Surface features' below) combined with a mass less than $5×10^{20}$ kg precluded Vesta from automatically being considered a dwarf planet under International Astronomical Union (IAU) Resolution XXVI 5.

I don't know the proper term to use here. Perhaps it is "hydrostatically relaxed" or something similar – I'm looking for the term that would suggest a body has reached a fairly round shape without major excursions, under the influence of its own gravity.

I think Ceres meets this criterion, and Vesta doesn't. Is Vesta the largest such body (by dimension or volume, not sure which would be the most reasonable measure)?


texts: What would the apparent size of the four stars in the Capella system be from the L1 point between the first pair?

texts: Capella is a system with four stars in two binary pairs (source: Wikipedia). If an observer were located at the L1 LaGrange point between the larger pair, how many degrees of the sky would each of the stars - both the first and second pairs - occupy?


texts: What abbreviations, letters, or symbols do we use to denote highly degenerate stars?

texts:
We have OBAFGKM to denote the strength of hydrogen lines.

Apparently we added S, N, C, and the W classes when we learned of new kinds of stars.

We use T, L, and Y to denote brown dwarfs.

The D classes refer to electron-degenerate stars (white dwarfs).

I've looked around quite a bit and I haven't seen anything regarding neutron stars and black holes. Does this mean there's no common symbol or abbreviation? What do they use in star catalogs? (I've never found a catalog with stars of all degeneracies in it.)


texts: Why does Saturn have both moons and rings?

texts:
From my understanding, a ring can form around a planet when a moon gets too close to its Roche limit, and gets ripped appart by the planet's gravity pull. That makes sense to me, but I don't understand why Saturn has both moons and rings at the same place.

I know the F ring is supposedly created by Enceladus' rejections, but I don't get why Pandora and Prometheus are not ripped appart as well as the ancient objects that formed the ring they are in.

I couldn't find specific explanations for this. Does it have any link to the moon's density?


texts: What is the difference between Sphere of Influence and Hill sphere?

texts:
Wikipedia's definition of Hill sphere is:

  An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere.

And then there is the sphere of influence:

  A sphere of influence (SOI) in astrodynamics and astronomy is the oblate-spheroid-shaped region around a celestial body where the primary gravitational influence on an orbiting object is that body. This is usually used to describe the areas in the Solar System where planets dominate the orbits of surrounding objects (such as moons), despite the presence of the much more massive (but distant) Sun.

To me, a planets Hill sphere and its sphere of influence appear to be the same. But the formulas to calculate the two sphere are different.

So what is exactly the difference between Hill sphere and the sphere of influence?


texts: Star formation around rotating black holes?

texts:
Please excuse an amateur question.
While trying to think of anything but what was happening during a dental procedure my mind turned to a model of a star close to a rotating black hole and the effects on the drawn in matter.

While it is obvious such matter would be excited to high temperature could the combination of rotation and excitation be enough to induce a sustained fusion reaction?

If so would this produce enough energy to maintain a fusion 'ring' at the event horizon - essentially a doughnut star?

Would there be enough of a reaction to start producing lighter elements?

Pure curiosity generated by an attempt to distract myself


texts: Correct units to use while plotting mollweide projection of sky chart with RA and Dec values, matplotlib

texts:
I am trying to plot a mollweide projection of a sky chart in Python using the matplotlib package. I am using the RA and Dec values from a database containing all stars in Hipparcos, Yale Bright Star, and Gliese catalogs (almost 120,000 stars).

This is the database: https://drive.google.com/file/d/1sJGUxQpnsw93q48I1v8qo6rbDKCeGYOH/view?usp=sharing

It contains the degree values of RA and Dec (column names are ra and dec respectively) as well as the radian values of RA and Dec (column names are rarad and decrad respectively).

My code assuming I need to use the radian values of Dec and RA is:

figure=plt.figure()
figure.patch.set_facecolor('black') #setting plot background to dark colour
ax = figure.add_subplot(111, projection='mollweide')
plt.scatter(df['decrad'], df['rarad'], s=1, color='red')
plt.show()
figure.savefig("weird.png")

And the output plot I get is:

My question is: Why are all the datapoints plotted within that circle only and not throughout the entire elliptical surface of the celestial sphere? Is it because I have chosen the wrong units (radians instead of degrees) or is the nature of the plot supposed to be this way based on the given star data?


texts:
Is the moon the most full only for one moment

As stated  here and here

Or does if stay the same full for a longer period of time (maybe because, the sun is grater then it, and light spreads out)

Full Moon Position of Sun


texts: Can a laser pointer ruin a long exposure photo?

texts:
I am a hobby astronomer and usually out with people that aren't familiar with the night sky.
In order to help them find constellations or show them stuff in the sky, I was thinking of buying a laser pointer.  Now because I do not want to ruin any photos of people that might be around my observation place, do some of the more experienced folks of you know about the effects of these laserpointers to photos?


texts: What is the degree of reflection of light from sun to the moon?

texts:
As Moon reflects the light from the sun it must have an ANGLE OF INCIDENCE and ANGLE OF REFLECTION...
What Is the angle of reflection then?


texts:
I always heard that mass is a measure of inertia during my school days. Is this  applicable to bodies which are in space ? Suppose there is space craft in a space; how would I define mass ?
I read some article and consulted with my teachers; they said mass is similar to conservation of momentum but I do not know how. And then he said mass is independent of frame it is constant, whereas I see momentum is itself a frame dependent quantity and velocity is frame dependent. I am confused how I would actually define a mass in space, what is the correct definition for a mass ?

I am high school boy, I will be thankful if anybody answer my question.


texts: If the VLT (very large telescope) can use interferomerty to act as a 200 meter mirror, then why are they trying to build the 39 meter E-ELT? What advantage does the E-ELT have, if the VLT can act as a 200 meter mirror?


texts: What about the magnetic field strength inside of an old accreting neutron star?

texts: As the magnetic field strength outside an old accreting neutron star is thought to be small (about $10^8-10^{10}$ Gauss), what about it inside the star? Because of the superconductivity and degeneration, can the strength reach $10^{17}$ Gauss or even more? Is there any article that describe such problem?


texts: How do we know that the discovered M87 black hole isn't just a star surrounded by a dust disk?

texts:
To an untrained eye like mine, pictures of stars surrounded by dust discs look very much similar to the picture of the M87 black hole.
Here are some pictures of these dust discs:

And here is a picture of the M87 Black Hole:

I find it quite difficult to differentiate between the two in a meaningful, non-hand-wavy way. What is the scientific explanation for the distinction? I'm not doubting the scientific explanation, I just don't understand it.


texts: Question about Telescopes , observations and explanations of the observations

texts:
Though I am active on Physics, I am new to Astronomy. I am an amateur astronomer, actually even worse than amateur. I have just started it.
I have a 60 mm objective 700mm focal length refractor which I mainly use with a 20 mm eyepiece.

I have a couple of questions regarding the telescope, observations and explanations behind observations. Since they are all interlinked I decided to ask them together.

1) So, firstly the basics. Regarding the resolution power of a telescope, we know from the Rayleigh criterion that diffraction does happen at the objective. But Diffraction happens when light passes through a narrow slit or bends around a corner (whose dimensions are of the order of the wavelength of light). But there is no slit and probably no bending of light. How come then do we assume a diffraction pattern to have formed when considering the derivation of the resolution of a telescope?

2) How are exit pupil and entry pupil different from the diameter of the eyepiece and objective?

3) Why on increasing the magnification does the brightness of the object reduce (the physics behind it)? I mean that if I use 4mm eyepiece then, first of all, it becomes very difficult to find the object and secondly its brightness gets reduced when compared to the brightness that I had got using a 20mm eyepiece?

4) Secondly why do the Barlow lenses reduce the brightness a lot. And why on using  Barlow lenses the field of view get smaller and the object passes twice as quickly as without the Barlow?

5) Now the biggest question. How is it possible that every time I point my telescope towards a visible planet or a star (I know it's a star because of the sky map), I see almost the same size, when stars are visible only as small dots. The stars appear to be bluish and the planets which I have seen (Mars and Jupiter) appear reddish. That's ok but a star can not be visible as a sphere. Stars appear as dots, don't they?

6)How can I make the telescope give the best results i.e which eyepiece should I use and whether I should use Barlow lenses or not?


texts:
I want to say first off, I am a complete beginner, starting out my interest in astronomy (so go easy!).

I have been able to see some nice sights on some nights, however, I had a strange experience the other night when viewing Saturn.

I had been viewing it the night before, using my highest magnification eyepiece and got some really nice views. It can be noted my telescope has a focal length of 1000mm and aperture of 100mm and I was using a 5mm eyepiece, which was really pushing the limits of the useable magnification, as far as I am aware.

So anyway, the next night had clear skies again, if anything maybe clearer. I was excited to see Saturn again, but this time once I had found it, I put in my 5mm eyepiece and just couldn't get it into the same focus I had the previous night. I googled it and somewhere said that although the skies can be good, the seeing can be poor.

What is meant by this statement and how am I able to tell if there is going to be a night with poor seeing in advance so I know which eyepieces I can use for that particular session? The app I use to check the skies only tells me if it is going to be clear so I don;t know what to be looking out for.


texts:
What's the orbital period of an asteroid which has synodic period of 429 days?

I know that the formula of orbital period for inner planets is $$P_{orb} = \frac{P_{syn}}{P_{syn} + 1}$$ and for outer planets is $$P_{orb} = \frac{P_{syn}}{P_{syn} - 1}$$ where $P_{syn}$ is the synodic period of the planet measured in years. However, I don't know whether the asteroid should be counted as an inner planet or outer planet.


texts: Flux vs. Velocity representation of spectra?

texts:
I'm looking for a basic explanation of the flux vs. velocity representation of a spectrum and how it's obtained from the regular flux vs. wavelength representation. A good example of this is in Arav et al 2001 (see here, Fig. 2 and 3).

More specific questions I can ask are:
• How do you convert between wavelength and velocity in these figures (i.e. what phenomenon are we using to make the conversion & what's the equation)?
• What thing has the velocity in question/what does the velocity represent? For example, is it the astrophysical object moving in this direction?
... But any further elaboration is much appreciated. As much as I've seen these (I'm trying to brush up, it's been a while since I've done astro stuff), I can't find any explanation for this; perhaps I don't know the right jargon, but yeah it's difficult to find a simple answer. Thanks in advance for the help.


texts: Is the Earth Really Spinning? (honest question)

texts:
Most people consider it common knowledge that the Sun's "movement" in the sky is only perceived due to the Earth's "spin and movement".

Based on this, you'd think that the stars in the night sky (when viewed with time lapse photography) would take a path in the sky similar to the sun. After all, both the sun and stars are practically stationary over the course of one day, given their accepted distances from the earth.

Yet, time lapse photography shows this is not the case.

Compare these photos:

1) Time Lapse of Sun

2) Time Lapse of Stars

3) Time Lapse of Moon

If the Earth's spin is the primary reason for "movement" of things (farther than clouds in the sky), then why do the sun and moon move through the sky in a similar fashion that is totally different from the sky-path of the stars? The Earth is said to spin around 360 degrees every 24 hours. It seems to me that everything in the sky should move in one direction where I live (Texas).

To me (in Texas), it seems the stars should sweep from horizon to horizon over the course of the Earth's night-spin (like the sun and moon do). Yet, the time lapse photos show circular paths in the sky for the stars. That would make sense at the poles, but not in Texas!?

-Lonnie Lee Best


texts: Amount of EM radiation (and particles) Earth "receives" from other planets in our solar system?

texts:
Can it be (or has it been already?) measured how much and what kind of EM radiation (apart from visible light)/particles Earth "receives" from a specific planet in our solar system?

Can other planets (giants like Jupiter/Saturn) also emit neutrinos or some other high energetic particles (like Sun)?


texts:
Some of brown dwarfs generate the enought amount of heat to warm their atmosphere up to 300 K. This is a condition to liquid water and, possibly, organic life forms.

Is where are a some studies about life forms in BD atmosphere?


texts: Has the microwave background radiation ever been blocked in order to prove it's really from the sky?

texts: A rather stupid question but I was just curious if the radiation could be virtual.  From the instrument....sort of like virtual particles. In other words if you just build a sensitive microwave detector and stick it in a South African Durban Deep gold mine 5 miles under the earth would it do the same thing?


texts: What could have this blue "bouncing" trail been?

texts:
Last night at around 8:45 PM (UK Time), I saw something I've never seen before. It was a cloudy night and I saw a flash in the sky. When I looked towards it, I saw something that looked like a shooting star, except it was much brighter than any I've seen before and larger. It had a blue/greenish trail and it was quite fast.
Whatever I saw, it started from the left and went to the right, then it suddenly went from the right back to it's starting position to the left. It repeated this about twice until it just disappeared. I was with my friend as we were going to do some stargazing but it was too cloudy. Neither of us has seen anything like this before, we were quite spooked by it.
What could we have seen?
Details: Hereford, UK. About 300° North West and about 50° above the horizon.


texts: Why do (most of) the planets rotate counterclockwise, i.e. the same way the Sun does?

texts:
Referring to the mechanisms explaining the solar system formation and to the initial rotation of the gaseous cloud that collapsed, I understand easily why the planets orbit the Sun the same way this one rotate (say counterclockwise) but I can't figure out why this apply to planets rotation too. Thinking about that from Kepler's laws and angular momentum conservation point of view, I might conclude that the planets should rotate clockwise because the velocity of the particles that aggregated during the planets formation was higher closer to the Sun...

Apart from a short explanation, I would like to have a good reference from the literature if possible.

Edit, to make my reasoning more explicit: following Kepler's laws, the particles that aggregate on the "day side" of the proto-planets in the east-west direction relative to the ground are faster than the ones hitting on the "night side" in the west-east direction. If we add all of these contributions, the planets should rotate in the opposite direction relative to the initial cloud (i.e. relative to the actual Sun rotation). I guess something is wrong or missing there (to counterbalance the phenomenon I just described) but I can't see what it is...

New edit: References I found some published articles dealing with this kind of question but I don't have the time right now to read them carefully. If someone is motivated to do so, do not hesitate ;-) If I find the answer to my question amongst these papers, I will post it there later. Of course, you may need to use the network of an institution with a subscription to these editors to access them:

R.T. Giuli (1968a) in Icarus: http://www.sciencedirect.com/science/article/pii/0019103568900821

R.T. Giuli (1968b) in Icarus: http://www.sciencedirect.com/science/article/pii/0019103568900122

A.W. Harris (1977) in Icarus: http://www.sciencedirect.com/science/article/pii/0019103577900793

J.J. Lissauer, D.M. Kary (1991) in Icarus: http://www.sciencedirect.com/science/article/pii/001910359190145J


texts: Astrophotography - Unable to achieve focus with current setup

texts:
I am new to astrophotography and am looking for guidance. I have the following equipment:

Orion StarBlast 9814 4.5" Altazimuth Reflector Tabletop telescope
1.25in Telescope Adapter Extension Tube T Ring for Canon DSLR SLR Camera DC618
17mm and 6mm lenses

When I use the lenses in the Telescope Adapter Extension, focus is not achievable. My theory is that the focal plane of most DSLR are 55mm back focus plane. Reflectors are positioned farther back than where the human eye would normally be placed, and hence it is usually difficult if not impossible to bring the camera into focus.

Is this true? Also, would a Barlow lens help achieve focus or be a waste of money? Any other suggestions?


texts:
I've been trying to find a clean definition what people mean by when they talk about diffuse ionized gas in the interstellar medium, but I couldn't find anything so far.
Apparently it's supposed to be trivial.

Is it simply ionized gas, mixed with non-ionized gas in the ISM through the process of diffusion?


texts: Is it possible to extend the life of a star?

texts: Is it true that when a star 'runs out' of its fuel, it hasn't really used up all its hydrogen, it still has hydrogen left, but the hydrogen is not at its center where the fusion happens. If something crashed into it rearranging its content a bit, could its life be extended?


texts: How much radiation would something need to output to sufficiently affect the CMB?

texts:
Let’s say that I had some device capable of outputting insane amounts of radiation, across the entire EM spectrum. If I wanted to affect the CMB to the point that, were Earth-based scientists to know that the device existed, they would be unable to gather any useful data from the CMB regarding the early universe, how much radiation would this theoretical device have to output? Peaking in what frequency? Where would be the optimal place to put it? Or multiple copies of it?

If someone who actually knows stuff about the CMB is scratching their head thinking I’m nuts, I probably am. I don’t know much about the CMB, except that it’s random EM waves that are leftovers from the Big Bang, peaking in the microwave frequency range. So if what I’m describing is absolutely implausible across all frequencies and amplitudes, I would appreciate if you could explain why, given my low level of knowledge on the topic.

In the event that for certain waves what I’m describing is possible, I would be curious to know if there’s actually anything in the known universe that could do it.


texts: How "fast" do astronomical events occur?

texts:
I've seen quite a few news about astronomy but I'm not deep into knowing how it works or how people observe it. It got me thinking: if I want to watch a supernova for example, how long will I have to look for it to watch the whole "event"? Or events like NASA "seeing something coming out of a black hole", were they "looking" at the blackhole when something came out really quickly or did they observe it for months? Or maybe the birth of a blackhole?

Now, I do know black holes that quite a few trillion of years to vanish, I'm just curious how much time does it take for astronomers to observe such events.


texts: Trying to understand the way Saturn's ring look in this famous Cassini image

texts:
You all probably know the famous The day the Earth Smiled image. For the life of me I can't understand what's going on with the rings there. Seems like the "bottom" half of the rings is the half that goes "behind" Saturn (relative to the camera), but then what's going on with the distortion of the rings on the upper half when they (presumably) cross in front of Saturn? I can't make sense of of this image.

Higher resolution and annotated versions available


texts: Can Pluto and Neptune collide anytime in future?

texts: We know that, the orbit of pluto ambiguously cuts the orbit of Neptune. I want to know, whether the revolving speed of Neptune is equal to the revolving speed of pluto. If not, is there a possibility in future,  that the two planets, or rather two celestial bodies will collide in future?


texts: I am going to ask an odd question, It is: In most of the images of our solar system we see that the planets are arranged in  a straight line. As the paths of the planets are elliptical, could it be possible for planets be arranged other than in straight line (a bit up and down) ?


texts: Available data on the Milky way around 1920

texts:
In a book by Alexander Moszkowski, there is an Einstein quote about a hypothetical size of the universe (100 million Light years). Moszkowski claims that Einstein had deduced that from the gravitational constant and the mass distribution of the Milky Way, as known around 1920. My question:

What was the data and the best estimate of the mass density of the Milky Way, back then in 1920?


texts:
How evasive is the Earth to Catastrophic Meteors?

Google says the Earth is approximately 92.96 million miles from the Sun. It also says the suns radius is 432,474 miles.

Therefore, it is 93,392,474 miles from the Earth to the center of the Sun:

92,960,000 + 432,474 = 93,392,474 miles

I've heard the Earth's orbit is elliptical, but to keep things simple, let's just say the orbit is a perfect circle. Therefore the radius of this circle is 93,392,474 miles. This would mean the diameter of our orbit would be 186,784,948 miles:

93,392,474 x 2 = 186,784,948 miles

The circumference of a circle is π x the diameter of a circle. Therefore, the number of miles in Earth's orbit would be about 586,504,736.72 miles:

3.14 x 186,784,948 = 586,504,736.72 miles

So, the Earth travels about 586,504,736.72 miles in one year. Since there are 365 days in year, that would mean that Earth travels about 1,606,862 miles in one day.

586,504,736.72 ÷ 365 = 1,606,862.292383562

That would mean the Earth travels about 66,953 miles per hour:

1,606,862.292383562 ÷ 24 = 66952.595515982

And this would mean the Earth travels about 1,116 miles per minute:

66952.595515982 ÷ 60 = 1115.876591933

Lastly, this would mean the Earth travel about 18.6 miles per second:

1115.876591933 ÷ 60 = 18.597943199

Google says the diameter of the Earth is 7,917.5 miles. So, as fast as the Earth is traveling, a catastrophic meteor has about a 7 minute window to hit target Earth.

7917.5 ÷ 18.597943199 = 425.719119329 seconds
425.719119329 ÷ 60 = 7.095318655 minutes

What are the odds of a catastrophic meteor hitting the Earth? Or, maybe a better question: What are the odds of the Catastrophic Earth hitting a poor meteor!?


texts: Are Dwarf Galaxies a Subset of Galaxies?

texts:
The question is a rather obvious one.
In normal English a word like "dwarf" used as an adjective is assumed to modify the following word.  Thus a "dwarf galaxy" would be assumed to be a galaxy of a type which is noticably smaller than galaxies of at least one other type.
And in science there might possibly be an established dividing line between dwarf galaxies and ordinary sized galaxies, just as there might possibly be a diving line between ordinary sized galaxies and giant galaxies.
But in 2006 the International Astronomical Union created new definitions, in which a dwarf planet is not a subcatebory of planet, but a member of a totally separate class of astronomical solar system object.  Dwarf planets are not included in planets, and planets do not include dwarf planets.
That is ungrammatical in English, but it follows a pattern.
A minor planet is mostly equivalent to what is usually called an asteroid.

A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any star with a planetary system) that is neither a planet nor exclusively classified as a comet.[a] Before 2006, the International Astronomical Union (IAU) officially used the term minor planet, but during that year's meeting it reclassified minor planets and comets into dwarf planets and small Solar System bodies (SSSBs).1

https://en.wikipedia.org/wiki/Minor_planet[1]
So in astronomy a minor planet is not a planet, and a dwarf planet is not a planet. The category of planets does not include minor planets or dwarf planets.
And thus it seems possible to me that perhaps in astronomy a dwarf galaxy might not be a galaxy.
If dwarf galaxies are a subcategory of galaxies, it would correct to call a dwarf galaxy a "galaxy" for short.  But if dwarf galaxies are a category of astronomical object separate from galaxies, it would not be correct to call a dwarf galaxy a "galaxy" for short.
So are dwarf galaxies a subcategory of galaxies, or are they considered to be a separate category from galaxies?


texts: How does the Hubble telescope capture long-exposure photos?

texts:
I recently learned (if I understood correctly) that the famous Ultra Deep Field photo(s) required something like a 10-day exposure on a very tiny piece of the sky to achieve. How can this occur when the telescope is constantly in motion zipping around earth at ~17,000 mph? Wouldn't the earth and/or possibly the moon get in the way every 90 minutes or so?

I suppose my question applies to just about any photo it takes since it would be out of alignment of the target in less than a second.


texts: What are the prerequisites for a meteorite to reach the ground?

texts:
I'm sure it's a function of atmospheric entry angle, meteorite mass (and possibly composition/density), and its velocity, but what function is it?

Above all, how does it depend on velocity? Will the slower meteors impact more often (less kinetic energy to dissipate) or the fast ones (less time spent in the atmosphere = less time to melt and evaporate all the way through)?


texts:
One method used for detecting exo-planets is to look for a slight dip in the parent star's luminosity as the planet transits the stellar disc. Intuitively, it seems to me that if planetary systems in our galactic neighborhood are randomly oriented, there would have to be a very large proportion of them in which transits can never happen from Earth's viewpoint. Perhaps, however, the assumption of random orientation is incorrect, and there is some alignment of the axes of rotation of planetary systems, which would facilitate detection of planets in some preferred plane (the galactic plane?).

In popular presentations concerning the search for exo-planets, I have never seen this issue addressed. What observations and/or assumptions are used in arriving at a realistic estimate of the number of exo-planets in our region of the galaxy?

(There are related questions in this forum, but I haven't found one that asks about the possible alignment of the axes of rotation.)


texts: I am confused about the definition of the period of an eclipsing binary, a binary configuration where one component crosses the other across the line of sight. When people say that an eclipsing binary has a period of T years do they mean that the period of the individual component is T years?


texts:
There are some pyramid shaped observatories around the world, like the Solar Lab Pyramid in the Canary Islands or The Pyramid International Laboratory in Nepal.

I was wondering, why a pyramid? Because of its relative stability, or is there something else?

below: Solar Lab Pyramid in the Canary Islands.

below: The Pyramid International Laboratory in Nepal.


texts: Is this object an asteroid or comet, and how can it produce so many tails?

texts:
The NASA Hubble Space Telescope Science Release HEIC 1320 - When is a Comet not a Comet? - Hubble astronomers observe bizarre six-tailed asteroid is self-explanatory, but it doesn't explain what actually happening.

Sometimes comets will show two tails, but one is usually identified with the path of the comet's orbit, and the other with the direction away from the sun. But this is an asteroid (or so it seems) and there are many tails with complex behavior.

edit: I've found this ArXiv preprint: The Extraordinary Multi-Tailed Main-Belt Comet P/2013 P5 where "main belt" refers to the main belt of asteroids, and yet the object is now called a comet.  Wikipedia lists both names and suggests both designations - asteroid or comet:

  311P/PANSTARRS also known as P/2013 P5 (PANSTARRS) is an asteroid (or main-belt comet) discovered by the Pan-STARRS telescope on 27 August 2013.(4) Observations made by the Hubble Space Telescope revealed that it had six comet-like tails.(5) The tails are suspected to be streams of material ejected by the asteroid as a result of a rubble pile asteroid spinning fast enough to remove material from it.(2)

It's been three point five years now - has this been figured out yet? Is it an asteroid or comet (or both), and how can it produce so many tails?

above: HST image of comet P/2013 P5 from here.

above: HST image (annotated) of comet P/2013 P5 from here.


texts: While this question could fall under Space Exploration or Physics (I don't have the "best" reputation on THAT forum), I feel that it is very much related to how the universe itself functions, especially regarding Einstein. While looking to see if anyone has confirmed our universe having a 4th spatial dimension (or some non temporal or spatial dimension) due to the fact that space curves, for me curvature requires a higher dimension (paper bending or a stick bending, always +1 to its dimension), I was met with the explanation that this "curvature" was intrinsic and not extrinsic (requiring another dimension). That was fine and all until I realized that wormholes would be exclusively extrinsic, how could you make a wormhole if bending space is intrinsic? Obviously you can't. But Einstein  put a lot of time into something that apparently wouldn't work, so personally I am siding with Einstein here. But that still leaves the question; Are wormholes possible thus making our universe a 3+1+1 dimensional universe (I wouldn't group it up with the 3 because that would say that matter could move like that, it can't) or are wormholes impossible and so our universe remains 3+1 dimensional?


texts: Why (the heck) is the basic angle of GAIA 106.5°?

texts:
This answer to Why does the Gaia space telescope have two main mirrors says:

According to the GAIA FAQs which does an excellent job: http://www.cosmos.esa.int/web/gaia/faqs:

Why is there an angle of 106.5 degrees between Gaia's 2 telescopes?
The choice of the so-called basic angle of GAIA was a non-trivial one. On the one hand, it should be of order 90 degrees to allow simultaneous measurements of stars separated by large angles on the sky. On the other hand, it should not be a harmonic ratio of a 360-degree circle (e.g., 60 deg, 90 deg, or 120 deg). Taking these considerations into account, acceptable ranges for the basic angle are 96.8 +/- 0.1 deg, 99.4 +/- 0.1 deg, 100.5 +/- 0.1 deg, 105.3 +/- 0.1 deg, 106.5 +/- 0.1 deg, 109.3 +/- 0.1 deg, 109.9 +/- 0.1 deg, etc. Accommodation aspects identified during industrial studies subsequently favoured 106.5 deg as the value finally adopted for Gaia.

I don't understand any of that.
Questions:

"On the one hand, it should be of order 90 degrees to allow simultaneous measurements of stars separated by large angles on the sky." Well any large angle is a large angle, why should it be 90?
"should not be a harmonic ratio of a 360-degree circle" I can imagine for instrumental reasons, but why are "acceptable ranges for the basic angle are 96.8 +/- 0.1 deg, 99.4 +/- 0.1 deg, 100.5 +/- 0.1 deg, 105.3 +/- 0.1 deg, 106.5 +/- 0.1 deg, 109.3 +/- 0.1 deg, 109.9 +/- 0.1 deg, etc." and what does "etc." mean here? Is there a pattern I'm not seeing?

Any angle with an uncertainty of +/- 0.1 deg can be reached with some rational number with a denominator of 1800 or less, that's as far as I've gotten.


texts: about H$_\alpha$ and H$_\beta$ lines in astronomical objects' spectra

texts:
Generally there may be some line ratio between the two lines.

If  H$\alpha$ is strong, it is quite possible we can see H$_\beta$ too.

Is it possible we can see H$_\beta$, but can not see H$_\alpha$? In what kind of environments can it happen?


texts: How does gas in accretion disks of supermassive black holes create orbital torques on smaller black holes within the disks causing them to migrate?

texts:
This answer to What enhances the capture and merge rates of pairs of small black holes orbiting around supermassive black holes? links to Migration Traps in Disks Around Supermassive Black Holes which includes the following sentence in the introduction:

Migration toward trapping orbits may be such a mechanism. Objects orbiting within differentially rotating disks exchange angular momentum with the gas around them as they orbit, which results in a torque, typically causing the objects to migrate.

Question: How exactly do density gradients in accretion disks of supermassive black holes create torques on smaller black holes orbiting within the disks? Is it through simple gravitational forces, GR effects (maybe some "spin-orbit coupling" thing?), aerodynamic drag, accretion by the small black hole in a velocity gradient, something else, or some combination of these?
One sentence in the linked answer is

These "migration traps" (Bellovary et al. 2016) are caused by torques exerted on the orbiting objects (in this case black holes) caused by gas accretion.

and I think it means that the orbital torque (torque on the small black hole about the SBH's center) comes from gas accretion by the small black hole. If this is the case, is it possible to elaborate a bit?


texts: Eyepieces and accessories recommendations for a Starblast 4.5-inch scope in a city as well as darker suburbs, and kid friendly?

texts:
For the Starblast 4.5 Telescope I currently have an Orion 7-21mm Zoom eyepiece attached on top of a 2x Barlow lens. I like them because of the magnification they give stacked together and the zoom eyepiece has a good eye relief which is very kid-friendly, and the zoom function is very convenient to find things quickly by zooming in and out without changing eyepieces or focus.
Are there other eyepieces or accessory (or better ones) I can buy for better viewing of the brighter Messier star nebulae and galaxies specifically under

a) a city environment with lots of light pollution
and/or
b) a darker suburb location?

Please specify if the accessory equipment is good for a) or b), or both. Thanks!
Here are the specs for the Starblast:

Aperture: 4.5 inch aperture
Focal length: 450mm
Focal ratio: f/4 focal ratio
Came with two Explorer II 1.25 inch Kellner telescope eyepieces (17mm and 6mm)

Thank you!


texts: Why don't (or can't) stars be more than 325 or so times the mass of the sun? What limits their size?

texts:
Is there a particular reason why stars cannot grow as massive as they want to?
And why doesn't this limit apply to supermassive black holes?


texts: Calculating right ascension for two planets and their separation using python

texts:
I am experimenting with the Python library PyEphem for astronomy, and I am trying to recreate/understand some of the basic calculations there.

There is a function called separation that, given two planets and a date/time, it calculates the separation (angle) between those 2 planets with respect to their "x" projection in a plane.

So for 2018/1/1 for planets Mercury and Mars, we have:

import ephem
import math

mercury = ephem.Mercury('2018/1/1')
mars = ephem.Mars('2018/1/1')

s1 = ephem.separation(mercury, mars)
print(math.degrees(s1))

Which returns

33.792384499568264

But if I wanted to calculate this without the separation function, then I think that the calculation would be as simple as the "right ascension" of 1 minus the "right ascension" of the other:

math.degrees(mercury.ra) - math.degrees(mars.ra)

Which returns

35.114532008671574

Why are the angles different? Since I am not including a latitude and longitude of the observer, all the calculations are supposed to be geo-centric, according to PyEphem.

Is anybody familiar with the calculations going on behind PyEphem, or another library with built-in ephemeris that can produce consistent results for separation?


texts: Working with high-magnification eye-pieces

texts:
My telescope (Newtonian reflector) has a focal length of 900 mm. So, using a 25 mm eye-piece, I get 900 mm / 25 mm = 36X magnification. Using a 10 mm eye-piece, the magnification would be 90X.

My problem is that I cannot see a thing with the 10 mm eye-piece. If I would insert the 25 mm eye-piece, find a bright star, and adjust the eye-piece distance from the secondary mirror, I would get the things ready after some trial and error. With the higher magnification eye-piece, it's just all black. What should I do to get around this?

The aperture is 130 mm if it matters.


texts: I recently read that water more than likely got here by comets (carrying water) hitting the Earth. However it also it says the impact of a comet hitting the Earth is much greater than an atomic bomb. So if comets brought water to Earth how was it not vaporized when the comet hit the atmosphere or in the massive explosion when it hit the Earth itself?


texts: Metalicity and age of bulge stars vs halo

texts: From what I understand of current models, the bulge of the galaxy formed first, and thus, would contain older population II like stars. Currently, however, the halo has a higher population of stars with lower metalicity than the bulge. Is this because the density  of stars in the bulge is much higher? My thinking is that this is a densely populated region of massive hot stars that die quickly and create a newer region of next generation population I stars that are metal rich. The inverse would be true for the halo where stars are less densely packed. Is this correct?


texts: Stars twinkle because their light has to squeeze through several different layers of the Earth's atmosphere. So why doesn't the moon twinkle as well?


texts: How long would it take to first detect a new object in the night sky given its apparent magnitude?

texts:
If an object as bright as the star Polaris (Apparent Magnitude +1.8) showed up in the night sky, it would be detected almost instantly by stargazers.  If an object as bright as Pluto (Apparent Magnitude +13.65) showed up, presumably it would take much longer for someone to notice (perhaps days or weeks?), since it is too dim to be seen by the naked eye.  For the faintest objects seen from Pan-STARRS 1.8 meter telescope (Apparent Magnitude +24) are we talking about years or decades to notice a new object?
Specifically, I am looking for a function that gives me something like the MTTD (mean time to detect) a new object in the night sky, given its apparent magnitude.  This could also be in the form of a graph.  Does such a thing exist?
I am asking this question to improve my answer to How early could we detect an asteroid the size of the one that caused the extinction of the dinosaurs?


texts:
Venus is often refered to as Earth-twin, simply because of its similar size. In the very hypothetical case of a human mission to Venus, and leaving the all other challenges of Venus toxic atmosphere aside, I am wondering how big the problem of natural radioactivity is on Venus (on cloud top level).
For Mars, I found a reference from MarsOne

Mars's surface receives more radiation than the Earth's but still blocks a considerable amount. Radiation exposure on the surface is 30 µSv per hour during solar minimum; during solar maximum, dosage equivalent of this exposure is reduced by the factor two.

I know that both Mars and Venus do not have a significant magnetic field which would shield away solar winds. Do I assume correctly that the radiation exposure of the solar wind is the main contribution to the "background" radioactivity on Venus?
References

Never Mind Mars, What About a Trip to Venus First?: Apparently there is serious proposal to send astronauts to a floating cloud city in Venus’s atmosphere before heading to Mars.
NASA: Solar Radiation Patterns That Expose the Moon
A related question about the solar radiation part: Why Venus evolved so differently from Earth?


texts: Does the gravity of the planets affect the orbit of other planets in our solar system?

texts: When one planet passes near another during its trip around the sun, does their gravitational pull is strong enough to disrupt noticeably each other's orbit ?


texts: What is the "lunar irradiance" received by the Earth from the full Moon?

texts: The solar irradiance received by the Earth is 1361 $W/m^2$. What is the lunar irradiance received by Earth from the full Moon in the same units? My first thought was to work it out via apparent magnitude but I don't think that would work out.


texts:
The other night, while playing Elite: Dangerous, I came across a rather strange celestial body - one I never imagined possible.

It was a Brown Dwarf star with a very large ring.

Is something like this even remotely possible? Have such objects actually been seen?

It was also quite interesting that this star wasn't even the primary star in the system. It was orbiting a larger star (blue-white, I think), along with several planets.


texts: Instantaneous gravity in equation of motion for ephemerides calculation

texts: The equation of motion used for the calculation of orbits of solar system objects (Eq(27) in https://ipnpr.jpl.nasa.gov/progress_report/42-196/196C.pdf ) is formulated in terms of instantaneous variables, that is the acceleration of an object is assumed to depend on the instantaneous positions, velocities and accelerations of all other objects in the solar system (this has been confirmed to me by the authors of this publication). Would this not mean that information travels infinitely fast and thus contradict Relativity?


texts: How can orbital resonance sometimes have a stabilising effect, whilst other times, it has a destabilising effect?

texts:
I have just started learning about orbital resonance. I understand how bodies in orbital resonance will line up according to the orbital ratio number, and there will be increased gravitational effects upon alignment.
However, I do not understand how resonance can often have a stabilising effect, while other times, it is destabilising? For example, in the Io-Europa-Ganymede system, the 4:2:1 resonance has a stabilising effect. But also, I read that the gravitational perturbances when the moons align cause them to have elliptical orbits, so is it really a stabilising effect? There are other examples where orbital resonance completely prevents stability, like in the Kirkwood Gaps of the asteroid belt.
So my question is, how do orbital resonances form in the first place, and what determines if they are stable/unstable? Thank you.


texts: So it was around 6 AM and i was looking at the sky and there were not many visible stars because the sun was about to rise. There were two stars that were brighter than the others and i was looking at them for about 5 mins. They weren't moving and the space between them seemed very little. Suddenly, one of those stars started moving down in direction towards the horizon and the other remained still. The star continued to go down. In 1 or 2 minutes it went behind the horizon and stopped being visible, while the other was still at the same place like earlier. At first I thought that the moving object was a satellite, but then i realised that a satellite would orbit the earth with constant speed, and the object i saw remained still for atleast 5 mins. So what else can it be???


texts: Why are all space observatories in Chile?

texts: Anytime I hear about astronomy there is observatory in Chile involved. Sounds like there are the best conditions to watch space. Anyone care to explain what is the reason?


texts: Why do people choose 0.2 as the value of linking length in the friends-of-friends algorithm?

texts:
The friends-of-friends algorithm (hereafter FOF) is commonly used to find halos in cosmological simulations. And FOF depends on only one parameter, linking length, $l_\mathrm{link}=b \left(\frac{V_u}{N}\right)^{1/3}$, where $V_u$ is the volume of the simulation box, and $N$ is the number of dark matter particles.
In most literature, the value of $b$ is set at 0.2. What does $b\approx 0.2$ mean physically?


texts: I read from somewhere that astronomer use X-ray to observe the "shaking" not to be confused with wobbling of a distant neutron star, I think it is similar to what geologist using microphone to detecting sound wave (vibration) inside the earth (no pun) to find out its composition. How does a tightly packed super dense object like neutron star generate starquake? is this neutron star signature move... i mean trait?


texts: What are these "nonstandard format" lines in Distant Artificial Satellites Observation circulars?


texts: How to shift an AGN X-ray spectrum to rest frame

texts: I have limited information to shift a spectrum (in the X-ray 0.5-10 keV bandpass) from redshift $z=2$ to the rest frame. I have a plot of normalized (photon) counts s$^{−1}$ keV$^{−1}$ as a function of energy (keV). Is there a way to shift the spectrum I have to rest frame? Or do I really need luminosity distance, etc.?


texts: Was there ever a hypothesis about a planet called Biga?

texts:
I just re-read a popular astronomy book that was published when I was a child, about 1973. It starts with an interview to Harold C. Urey where the interviewer asks about a hypothetical planet in our solar system called Biga.

Has a hypothetical planet ever been called "Biga" by its  proponents, in the same way there was a proposed planet "Vulcan", or is this "Biga" name a printing or translation error?

If it's a mistake and there never was a "Biga" proposal, which is the proposed planet with the most similar name?

This link contains such interview, in Spanish. I will translate and quote its most relevant text:

  Interviewer: At present, do we know all the planets of the Solar
  System, or is there the possibility of still unknown planets in our
  system?

  H.C. Urey: We certainly know the nine usual planets. Recently there
  has been much talk of another object moving in space and supposed to
  be close to the Solar System, but I am not sure that it is a real
  hypothesis, and personally I think it is better to wait before
  deciding whether it is Not of a planet. I consider it sufficient to
  say that there are Mercury, Venus, Earth, Mars, Jupiter, Saturn,
  Uranus, Neptune, Pluto and the asteroidal belt.

  Interviewer: When talking about this object, are you referring to the
  so-called planet Biga?

  H.C. Urey: Yes, that's what I meant, but I'm not sure of its
  existence.

Please, remember:

1- This interview mas made about 45 yeras ago.

2- I'm not asking if this hypothesis is correct or even possible. Only want to know more about this hypothesis which was well known by H. C. Urey in the 70's.

3- I'm not asking if the name "Biga" is a correct proposal of a name for a planet in our Solar System accordind to current naming rules.


texts: Do other planets in our solar system experience eclipses or is this unique to Earth?

texts:
Here on Earth we get to experience lunar and solar eclipses. Is this pure coincidence that everything happens to line up?

Do other planets in the solar system experience this phenomenon or is this something unique on Earth?


texts: What does G/IG signify in the galaxy names ESO 012- G 014 and ESO 293-IG 034?

texts:
The NASA/IPAC EXTRAGALACTIC DATABASE (NED) lists galaxies from the European Southern Observatory (ESO) catalog as  G  (ESO 012- G 014) or  IG  (ESO 293-IG 034).

What does G/IG signify?


texts: Dependence of shape of stars and gas in galaxies on dark matter halo

texts:
I have studied about galaxies having different shapes. But do the stars and gas in galaxy have different geometrical shape separate from each other. What factors decide this?
I think that the mass of dark matter halo affects these shapes but how? Is there any relation connecting these two?


texts: Have I misunderstood Hipparcos Data with Arcturus?

texts:
If I take Arcturus for example, its HipID is 32349 and when i look at the raw Hip_Main.dat, it stays that the star's Spectral is K2IIIp whilst Wikipedia says the star's is KOIII.  Why the discrepancy, have I misunderstood the Hip data?

Mu Cephei is M2e Ia, in Hip its m2Ia.

Achernar is B6 Vep whereas in Hip its B3Vp

Zosma for example, matches, I get A4V in the Hip file and it says A4 V in Wiki.


texts: How could time only have started with the Big Bang?

texts: I understand that before the Big Bang time (as well as dimensions) didn't exist. But how could this be? If there was no time then nothing could change, and so time itself couldn't come into existence.


texts: Why is space and time spoken of as one thing

texts: I've never understood why it's spacetime and not space and time. In other words why aren't space and time spoken of as two different things. Can someone explain to me why this is


texts: Are Sun-Earth L1 & L2 points generally considered outside the Earth's Hill Sphere?

texts:
This image from Wikipedia of the SE Lagrange points and the Hill Spheres suggests that the SEL points are outside the Earth's Hill Sphere. (The Hill spheres are the circular regions surrounding the two large masses.)  Is this correct?

But I thought I've read that both the Hill Sphere and L1 & 2 are 1.5 million kilometers from the Earth.


texts: Can a telescope ever increase the apparent luminance of an extended object?

texts:
From what I know about common telescope designs, telescopes don't increase the apparent luminance of extended objects compared to the luminance seen with the naked eye. In this sense extended objects don't appear "brighter" (per unit solid angle of object/image), although the total light flux received from the object (the illuminance) can increase due to the increased magnification (the object looks bigger through the telescope). The best that can be done is keep the luminance the same, which requires eliminating transmission losses. In addition to transmission losses, the light received by the eye per unit solid angle of image is further reduced at high magnifications when the exit pupil is smaller than the viewer's pupil.
Can there be no telescope design (purely optical, e.g. without using electronic eyepieces) that makes extended objects appear "brighter" (in the above sense) than with the naked eye, by somehow overcoming the limitation of the observer's pupil? If so, can this be proven? If not, what would such a design look like?


texts: Why do we believe that the super massive black holes at the centers of two merging galaxies would themselves merge?

texts:
When listening to podcasts or watching youtube videos of astronomers discussing galaxy mergers, I often hear talk about how the super massive black holes at their centers will themselves merge during or shortly after the collision. Why do we believe this to be the case?

A priori, I would expect SMBH's to behave the same as all other galactic objects. They might be extraordinarily massive, but physically they are still minuscule compared to the vast empty space between stars in a galaxy. Collisions between objects (not counting gigantic clouds of gas and dust) would be extraordinarily rare, so why do we make an exception for the SMBH's?

I could see a case for them merging in the (rare?) instance where the host galaxies hit each other in such a way that the mutual center of mass happens to coincide with their individual centers of mass. In that case, the SMBH's could be close enough to orbit about one another, losing energy to gravitational waves and eventually merging. I would think that this scenario is rather rare, however. I find it more plausible that an average galaxy merger would leave the SMBH's independently orbiting the center of the combined galaxy, too far apart to lose any significant kinetic energy to gravitational waves.

The astronomers who talk about galactic mergers know a heck of a lot more than I do about the subject, so I presume there are flaws in my assumptions or my understanding of the physics. What am I missing?


texts: Does bending of light around the Sun depend on the wavelength?

texts: If the energy of light is high, does its curvature differ from that of low-energy light around the Sun? In other words, if the wavelength of the light is shorter than another wavelength of light, then does the bending of the two lights differ around the Sun?


texts: The formation of new stars as depicted in Feynman's lectures

texts:
I've stumbled upon this curious passage addressing the formation of stars in Feynman's lectures on physics:

  Whether we have ever seen a star form or not is still debatable.
  Figure 7–12 shows the one piece of evidence which suggests that we
  have. At the left is a picture of a region of gas with some stars in
  it taken in 1947, and at the right is another picture, taken only 7
  years later, which shows two new bright spots.

While I fully agree with Feynman's later note† that it's extremely unlikely that this picture shows stars forming, what does it actually show?

There is no reference to it, and no mention of filter used, the physical scale or even of the celestial object shown, and I've gotten rather curious as to what the seven-year change represents. Variable stars? Or is the displacement due to proper motions? Film artifact? Something else entirely?

Figure in question (Source):

† Feynman concludes the paragraph with "[has the star formation somehow occurred?] Perhaps, and perhaps not. It is unreasonable that in only seven years we should be so lucky as to see a star change itself into visible form; it is much less probable that we should see two!" - indeed, the star formation rate of the whole Galaxy would be of this order!


texts: There's a theory that the reason Mercury has such an enormous iron core is that it was once a much larger planet before it got impacted by an object, resulting in most of its mass getting blasted away. If this theory is true, how large was Mercury before the impact? Could it have been the size of Earth or Mars, or maybe even a super-earth?


texts: Stopping down a telescope to reduce brightness by 5 magnitudes

texts:
Suppose we have a variable star with a change in magnitude between maximum and minimum of 5 mag.

How much would you need to stop down a telescope (block some of the area) so that the star at maximum brightness appears to be the same brightness as when it is at minimum magnitude with no stop.


texts: Why did Earth need material from comets to assemble the raw materials for life?

texts:
It's often speculated that matter from comets played a crucial role in the development of life on Earth. For example, Maps of Time says,

  Many of the ingredients of the early atmosphere (including much of its water), together with many of the organic chemicals that formed the first life forms, may have been brought to Earth by the comets that bombarded its surface for the first billion years of its life (~Kindle location 1922).

Why the presumption that these materials must be brought by comets? Did Earth even have the right atoms to make water and those organic chemicals (hydrogen, oxygen, carbon)? And what are comets doing with heavier elements in the first place? My understanding is that comets are (for most of their lives) very far from the run, and my impression was most of the elements other than hydrogen and helium and are concentrated close to the sun, with T Tauri Wind having push hydrogen and helium further away.


texts: Will intergalactic travel be impossible in roughly 15 billion years?

texts:
Even if we (or aliens somewhere in the universe living at that time) somehow invent something that can go just short of the speed of light, will intergalactic travel still be impossible in roughly 15 billion years?  Another way to put it, will even the nearest galaxy be receding from us faster than the speed of light in roughly 15 billion years?  To me, that would seem to require dark energy to increase dramatically, but seems to be the conclusion after thinking on the following points (although the expansion of space itself probably alters this estimate significantly):

In some further thought on the premises I posed in this linked question, I came to the realization that the universe may be "rapidly" changing:
Is there a physical limit to how far we can go?

Edit: This is different than that question in that I am focused now on the future state of our universe (when - the focus of this question) vs. the location (where - which was the focus of the prior question).

Known facts:

There is a part of the universe, about 15 billion years away, that is receding beyond the speed of light, and hence we can never reach.
If we were to pick a direction, there are a finite number of galaxies between here and that 15 billion light year position, (and I therefore deduce at some point, we would not be able to reach the next galaxy.)
Space is fairly uniform, so similar conditions should be present on a large scale
It would take at least 15 billion years to reach that edge, not accounting for space expansion.

My thought / question with the above in mind, does this mean that if we were to keep traveling, it is inevitable that we will eventually pass the last galaxy we can reach, and then all galaxies would be out of reach (except possibly the last one)?  Does this mean that in 15 billion years (then adjust for the expansion of space) that we won't even be able to reach the nearest galaxies, even if we find a way to travel at the speed of light?


texts: How does the Moon's gravity affect Earth's oceans despite Earth's stronger gravitational pull?

texts: Given that Earth has a much stronger gravitational pull than the Moon, how does the Moon have any influence on Earth's oceans?


texts: Which astronomical objects do we have images of, taken at ground level?

texts:
I am looking for images of astronomical objects taken at the ground level of the actual object, by something on the actual object, i.e. the Mars Rover.

Is this the complete list and accurate?.

Earth
Mars
Titan
Venus
67P/Churyumov-Gerasimenko
Moon

(I am not interested in images of objects take from the ground level of Earth).


texts:
I've never seen this Hubble picture, is it an artist impression? It's fascinating because there are voids apparently cause by supernovae.

From the video 1 Hour of Epic Space Music: COSMOS - Volume 1

The deviant art mentioned in the information below the video is by Natasha Sharma, she has a different style than that. There are also real photos from hubble in the video, I can't tell if it is a genius space artist or a real photo.


texts: Lightkurve from fits. file to csv - AttributeError: 'str' object has no attribute 'to_frame

texts:
I am trying to convert a TESS file into a csv file, using the to_csv function within the lightkurve package.
I have tried:
import pandas as pd
from lightkurve import TessLightCurveFile
import lightkurve as lk
lc=TessLightCurveFile(r"C:\Users\me\documents\myTESSfitsfile")
lc
pd.DataFrame.to_csv('lc');

which returns
AttributeError
 Traceback (most recent call last)
<ipython-input-12-64856d3ed63a> in <module>
      5 lc=TessLightCurveFile(r"C:\Users\me\documents\myTESSfitsfile")
      6 lc
----> 7 pd.DataFrame.to_csv('lc');

~\anaconda3\lib\site-packages\pandas\core\generic.py in to_csv(self, path_or_buf, sep, na_rep, float_format, columns, header, index, index_label, mode, encoding, compression, quoting, quotechar, line_terminator, chunksize, date_format, doublequote, escapechar, decimal)
   3176         """
   3177
-> 3178         df = self if isinstance(self, ABCDataFrame) else self.to_frame()
   3179
   3180         from pandas.io.formats.csvs import CSVFormatter

AttributeError: 'str' object has no attribute 'to_frame'

I have also tried:
lk.lightcurve.TessLightCurve.to_csv('lc');'''

after I have flattened, folded, bls etc the light curve which also returns the same error. I know that 'to_frame' is what is missing from my 'str' and that this is probably really basic and 100% user error/me doing something stupid but I cannot figure out how to fix it.


texts: How well do planetary orbits fit with Johannes Kepler's in- & circumscribed Platonic solids?

texts:
In Mysterium Cosmographicum (1596) Johannes Kepler proposed that the relative distances between the orbits of the six ancient planets (six because heliocentrism had recently added Earth as one of the planet) correspond to the geometry of the five Platonic solids. Each planetary orbit was assumed to be the grand circle of a sphere. Each Platonic solid would circumscribe an inner planet's orbitally defined sphere, and in same position inscribe the next outer planet's "orbital sphere".

How well does it fit?
Is it somehow mathematically related to the equally spurious Bode's law?
I would appreciate some hints or links to the geometric calculation itself, for me to compare with my own exercise trying to solve part of it.

Johannes Kepler proposed this order of matching planets with Platonics (I suppose because this order gives the best fit):

Mercury <-- planet

octahedron <-- Platonic solid

Venus

icosahedron

Earth

dodecahedron

Mars

tetrahedron

Jupiter

cube

Saturn


texts:
Will we be able to see our galaxy if we travel far enough? When we leave earth far enough, we'll be able to see Earth from distant, is it same with the galaxy? if we travel far enough, will we see the shape of our galaxy with naked eye?

Do we have to travel really far to see our galaxy like a dot? or just far enough to realize the size and shape of our galaxy?


texts: Would a satellite in geosynchronous orbit between the earth and moon track across the sky together?

texts: On Nov. 25 at 2:30a.m. from southern California while viewing the moon with a 70mm D x 400mm F telescope a point of light was seen at about the 7 o'clock position below the moon. As viewed the point of light was clear and seemed to be tracking with the moon. As I tracked the moon the point of light stayed within the same distance and in view. If it was a planet or star wouldn't they track slightly different than the moon ?


texts: A day in earth, a thousand years somewhere else

texts:
By followig the laws of physics, is it possible, that in a place far far away that one day (at that place), is counted as 1000 years in the earth.

I am familiar with black holes having impact on time. But would it be possible without black holes and such,

I think i've heard that the gravity has impact with the time too?


texts: How is IR spectroscopy used to determine the composition of asteroids?

texts: I am trying to understand how spectrometers or spectroscopy can be used to calculate the surface composition of asteroids for the purposes of asteroid mining.


texts: How to deal with the celestial meridian?

texts:
The book “Foundations of Astrophysics” by Ryden and Peterson states that one measures azimuth in the horizontal coordinate system eastwards starting on the northernmost point of the horizon circle. Thus, east becomes 90 degrees, et cetera. Wikipedia and other sites back this up.

However, it seems as if the position of the vernal equinox is always given relative to the southern part of an observer’s celestial meridian. During the vernal equinox, the sun culminates in the south, setting the siderial time to 0h0min. Knowing the difference between the local siderial time and a star’s right ascension gives you the current position of a star, relative to the southern half of the celestial meridian.

This website describes this too: http://www.polaris.iastate.edu/NorthStar/Unit4/unit4_sub2.htm

Did I miss something? We so explicitly defined azimuth as being relative to the most northern point on the meridian. Now we’re doing all our measurments relative to the south. I’m not complaining: stars culminate in the south, so this makes it easier. But where did we switch? And when must I switch? Did I make a mistake somewhere?


texts:
I read that a black hole can sometime "choke" on a star:

  "...the disrupted stellar matter was generating so much radiation
  that it pushed back on the infall. The black hole was choking on the
  rapidly infalling
  matter."

Why does a black hole choke? How do astronomers manage to observe this rare event?


texts:
If all of Earth's oceans were suddenly moved into outer space, how long would it take for them to evaporate?

My understanding is that water evaporates straight away in space... but what about when it's a quantity as large as an ocean?

Would it be a matter of seconds? minutes?

Would it be possible to swim in an ocean, that was floating in space, for a short while before it all evaporated?


texts:
Science fiction seems to depict the experience of traveling through wormholes (if possible of course) as seeing psychedelic colors or lines of light where stars once were (ie. 2001 or Contact). I wonder if theorists have any idea of what, if anything, the traveler would see.
This question may be subjective, but I know that there are theories about what an observer would see if traveling near the speed of light.


texts: Is there a chance that UFOs could be messages from other planets?

texts:
Description

Lots of people have thought to of seen UFOs (Unidentified Flying Objects), in fact, more than 83,000 have been reported this year alone. (and it's only February!) But most of the time it's just a shooting star or an airplane. But some of the sightings don't seem to be any of the things a UFO is commonly mistook for, but there isn't any evidence supporting if it is really a UFO. I might not be alone on this theory, but I believe the sightings that potentially could be UFOs are messages from the cosmos.

Question

There may not be much to back up my theory, but I just wish to learn some background knowledge:

  Are UFOs (and I guess all the UFO sightings reported, too) messages from another planet?

I also believe that if they are messages, they could possibly be responses to NASA's Voyager spacecrafts, for their purposes were to inform any forms of life out in space what earth is like with the 'Sounds of Earth' music disk attached to it.


texts: Why not use only one support vane for the secondary mirror, to avoid multiple diffraction spikes?

texts:
Each vane holding the secondary mirror in front of the primary causes a diffraction spike. Couldn't the secondary mirror be held with only one vane?

If stability is the issue, couldn't it be substituted be steering the vane to hold the secondary in place with precision, similar to active optics, regardless of winds? And why does the James Webb Space Telescope use three vanes when it will operate in undisturbed microgravity and shadow?

(Maybe the star spikes are desired 'cause they make stars look nice?)


texts: Whats the deal with black holes and "no information from inside the event horizon can leave"?

texts:
I don't know if this would fall under the other questions but I have always been confused by this... Information has left the black hole; its mass, rotation, location, and a few other properties. My problem has been that a true singularity should collapse the space around it and become separate from the universe as being able to observe the effects of a singularity breaks the fact that it is a singularity.

If I see a black hole I can already tell its mass and be effected by the gravity of an object which I can receive no other information from. This would break things too!

Let's just say I am an all powerful being who can do almost impossible things, but I still have to follow the laws of the universe. I decide to create a gravity observation station in orbit which uses lasers to see very small changes in the black hole. It also has a teleporter of sorts.

I then enter the black hole and construct another station, this one however uses small black holes and impossibly powerful thrusters and materials to do certain things. Its purpose is to manipulate and sustain a small black hole "orbiting" the inside of the bigger black hole (being sustained, barely, by the station) it then changes the direction this smaller black hole orbits in a kind of binary way.

I then step into a "teleporter" converted to binary instructions and encoded into these gravitational and rotational information. (Let's just say the station has no lifespan) and this station over a huge amount of time begins to piece together this code, and reconstructs me in the "teleporter" on board.

Have I just circumvented the event horizon using disclosed information available to me? Why would or wouldn't this work because for what I see being able to describe a black hole violates the properties of the black hole.


texts: Does creation of galaxy depend on black holes?

texts: I've read many times that every galaxy contains a black hole at the center. So is the creation of a galaxy linked to black holes?


texts: How to visually tell the difference between a planetary nebula and a supernova remnant?

texts:
If we see a nebula by looking through a powerful telescope, how can we tell whether we are looking at the remnant of a supernova or at a planetary nebula?
Thanks


texts: What causes an asteroid to loose control over it's orbit and enter in to another objects atmosphere?


texts:
Given a state of an ideal 3 body system (i.e., without external interference) in time $t$: the velocity $v_{i,t}$, mass $m_{i,t}$ and position $x_{i,t}$ for $i\in \{1,2,3\}$, using numerical method it is possible to determine any state in time $\hat{t}$, where $\hat{t}>t$. But is it possible to determine a unique state in time $\bar{t}$ where $\bar{t}<t$?

In another word, is it sufficient to know a single state of the system for any given time to deduce the states of the system for all time?

Equivalently, will two different states of the 3-body system results in the same state (maybe at different time) in the future?


texts: What kind of effects can maintain Uranus' rings' eccentricities?

texts:
The recent paper Thermal Emission from the Uranian Ring System has been in the news recently. The introduction mentions:

The ε ring, the brightest and most massive of the narrow rings, was shown to maintain an appreciable eccentricity (e = 0.00794) and an azimuthally-varying width; the ring is five times wider and ∼2.5 times brighter in reflected sunlight at apoapsis than at periapsis (French et al. 19881; Karkoschka 2001a2). However, many fundamental parameters about the ring system remain unknown, including the filling factor, composition, thickness,
1French, R. G., Elliot, J. L., French, L. M., et al. 1988, Icarus, 73, 349, doi: 10.1016/0019-1035(88)90104-2
2Karkoschka, E. 2001a, Icarus, 151, 51, doi: 10.1006/icar.2001.6596

I've read in Wikipedia's Rings of Uranus that several rings are expected to be fairly young, and there may be interaction with some small moons, but I am surprised that the rings could maintain a significant eccentricity, and do not circularize or smear out due to collisions.
Question: What kind of dynamical effects can maintain Uranus' rings' eccentricities?
Uranus itself has a huge J2 and so any ring that isn't perfectly equatorial should have an eccentricity-dependent precession. The paper states that the apoapsis of the rings is five times wider than the periapsis, so there's definitely a population of different eccentricities and semi-major axes within the ε ring.
List of rings and their orbital parameters


texts: What is the share of stars in total radiation input of Earth?

texts: Earth gets its radiation input primarily from Sun, then from reflected sunlight from Moon, and stars. Among these, what is the share of stars (or sources outside solar system) to this total radiation input of Earth. Is it negligible compared to solar radiation? (I am referring to top-of-atmosphere situation.)


texts: What are the challenges in finding Earth-like planets via the radial velocity method?

texts:
[I have re-asked this, because it was a good question by @Banyan, which was deleted whilst I was composing an answer.]
Most exoplanets that are found by the radial velocity ("Doppler wobble") technique are quite massive - Jupiter/Saturn-like objects.
What are the principle challenges in extending this technique to finding Earth-like exoplanets?


texts: What percentage of the hydrogen today has never been in a star

texts: It stands to reason that some of the hydrogen and helium that formed directly as a product of the big bang might never have fallen into a star to re-ejected when that star explodes.  My question is, given the best theory, what percentage of that matter has managed to drift without being sucked into a star.  Do we have any idea?


texts: How does one determine the effective temperature of a star from its spectrum?

texts:
Determining effective temperature of a star is in general a non-trivial task. Simple reason for this is that we can only study the electromagnetic radiation from a star, but not the temperature directly. The complexity is due to the fact that the radiation is produced in stratified stellar atmospheres, which are only partially characterised by stellar temperature, but also by many other factors, such as stellar mass, elemental abundances, stellar rotation, etc. What is more, the temperature of atmospheres varies with depth, whereas effective temperature is just a number.

From the other hand, temperatures and magnitudes are the most important quantities, characterising stars.

So, the question: How exactly does one use the spectrum to extract the information about the temperature of a star? By temperature here I mean effective temperature, or even the temperature profile of the atmosphere.

Note: This a rather textbook question. I created it because I have encountered a good existing answer by @Carl, previously posted in a bit less textbook discussion How well can we in principle determine $T_{\textrm{eff}}$ of a star? . This question seems to be a much better place for the answer.


texts:
What are the percentages of systems that have x number of stars in them?

What I have found thus far is something like:

Single Star Sytems = 69%
Double Star Systems = ~10%
Triple Star Systems = <20%
Quadruple Star Systems = ???
Quintuple Star Systems = ???
Sextuple Star Systems = ???
Septuple Star Systems = ???

Also... Is there any way to get these percentages for systems that have primaries that are Giants (more than 2 Solar Masses) Major Stars (0.5 to 2 Solar Masses), and Minor Stars (0.5 Solar Masses... red dwarfs)

I have found an article that says...

Giant Stars = 80% have companions
Major Stars = 50% have companions
Minor Stars = 25% have companions

Sources:
http://www.space.com/1995-astronomers-wrong-stars-single.html
http://boards.straightdope.com/sdmb/showthread.php?t=499261
http://www.skyandtelescope.com/astronomy-news/surprise-most-star-systems-are-single/


texts: The deduction of the Hill Sphere formula

texts:
This question may be a little lazy, but can anybody give me a proof of the Hill sphere formula? Acording to wikipedia, the formula for the radius, $r$, is

$$r\approx a(1-e)\left(\frac{m}{3M}\right)^{1/3}$$

where a body of mass $m$ is orbiting a much more massive body of mass $M$ with a semi-major axis $a$ and eccentricty $e$.


texts: Why is the flux density and amplitude different for galaxies than stars?

texts:
This might be a simple question, but I'm having a hard time answering it myself - or at least answer it correctly, I think.
I'm sitting with different spectrals; many for stars and one for a galaxy. The one for the galaxy has a flux density of 1e-13, while the stars range from 1e-7 to 1e-12.
My question is: why is galaxy's flux density lower than even the lowest stars? The galaxy also have very characteristic emission peaks. How is that reflected in the spectrum of the stars, or is that another phenomena?
To clarify, I am talking about a plot like this. I had to scale the stars spectrum down by a lot in order to match the galaxy's spectrum. Why is that?
Also why can't the stars produce the same spiky amplitudes as the galaxy? Is it because of the galaxy emission lines from billions of stars and vast amounts of dust and gas?


texts: How much more atmosphere is the sun passing through at sunset as opposed to at its peak in the sky?

texts:
I was looking at the sun during a sunset earlier today and wondered how much atmosphere you need to be able to look directly at the sun without damaging your eyes. The thought crossed my mind because I was continuously driving into an extremely oppressive sunset, my eyes were battered by the sun continuously for about a half an hour. Then, suddenly, the sun became more and more dull as it lowered into the sky.

My question is:

What is the approximate ratio of atmosphere between the sun and me, when setting as compared to when its positioned at a peak point in the sky? If you have the math behind an approximation like this from a topological observer, I would love to see that worked out.


texts: Which solar system objects have the least stable solar orbits?

texts: My research is not giving me anything definitive. Are the Apollo asteroids orbits less stable than the Centaurs or any Trojans, or is another class of objects in less stable solar orbits? On what timescale are those orbits unstable?